<html xmlns="http://www.w3.org/1999/xhtml" xmlns:math="http://www.w3.org/1998/Math/MathML">
<head>
<script>
function boom()
{
  document.getElementById("mfenced").setAttribute("mathbackground", "yellow");
}
</script>
</head>

<body onload="boom();">

<math:mfenced id="mfenced" />

</body>
</html>
